I've also read that Linus was pretty influential in BitKeeper design too. As in Larry McVoy would sit down with Linus to talk about the design of it. Git's core data structures had probably been bouncing around in Linus's head for months before first line of code was written.

If you know both BitKeeper and Git internals, you can think of their data models as near identical, with BitKeeper implementation using text files and SCCS, and Git using from-scratch data structures.